1. Field of the Invention
The present invention relates to a method and an apparatus for drilling a well. More particularly, the invention relates to a method and an apparatus for drilling a well in an underbalanced condition. More particularly still, the invention relates to a method and an apparatus enhancing safety of the personnel and equipment during drilling a well in an underbalanced condition using a dynamic column of heavy fluid.
2. Description of the Related Art
Historically, wells have been drilled with a column of fluid in the wellbore designed to overcome any formation pressure encountered as the wellbore is formed. In additional to control, the column of fluid is effective in carrying away cuttings as it is injected at the lower end of drill string and is then circulated to the surface of the well. While this approach is effective in well control, the drilling fluid can enter and be lost in the formation. Additionally, the weight of the fluid in the wellbore can damage the formation, preventing an adequate migration of hydrocarbons into the wellbore after the well is completed. Also, additives placed in the drilling fluid to improve viscosity can cake at the formation and impede production.
More recently, underbalanced drilling has been used to avoid the shortcomings of the forgoing method. Underbalanced drilling is a method wherein the pressure of drilling fluid in a borehole is intentionally maintained below the formation pressure in wellbore.
In underbalanced drilling operations, a rotating control head (RCH) is an essential piece of wellhead equipment in order to provide some barrier between wellbore pressure and the surface of the well. A RCH is located at the top of the well bore to act as barrier and prevent leakage of return fluid to the top of the wellhead so that personnel on the rig floor are not exposed to produced liquid and hazardous gases. An RCH operates with a rotating seal that fits around the drill string. The rotating seal is housed in a bearing assembly in the RCH. Because it operates as a barrier, the RCH is often subjected to high-pressure differential from below. In order for the RCH to work properly, stripper rubber elements designed to seal the drill pipe must fit around the drill pipe closely. These rubber elements are frequently changed on the job with new elements to ensure proper functioning of the RCH. However, even with frequent change of these elements, operators are often concerned about the safety on the high-pressure wells, especially where hazardous gases are expected with the return fluid. Additionally, in relatively high-pressure gas wells the use of drilling fluid density for controlling return flow pressure lowers production from the well and requires the produced gas be recompressed before it is fed into a service line or used for re-injection.
In another form of underbalanced drilling, two concentric casing strings are disposed down the wellbore. Drilling fluid is pumped into the drill string disposed inside the inner casing. A surface RCH is connected to the drill string at the wellbore. Another fluid is pumped into an annulus formed between the two casing strings. Thereafter, both of the injected fluids return to the surface through an annulus formed between the drill string and inner casing. Gas rather then fluid may be pumped into the outer annulus when drilling a low-pressure well to urge return fluid up the annulus. Conversely, when drilling a high pressure well, fluid is preferred because the hydrostatic head of the fluid can control a wide range of downhole pressure. The operator can regulate the downhole pressure by varying the flow rate of the second fluid. This method has a positive effect on the rotating control head (RCH) in high-pressure wells because the pressure of returning fluid at the wellhead is reduced to the extent that there is added friction loss. However, the RCH is not isolated from produced fluids therefore imposes a safety risk on rig operators from leakage of produced fluid due to a failure in the RCH.
A mudcap drilling system is yet another method of underbalanced drilling. This drilling method is effective where the drilling operator is faced with high annular pressure. FIG. 1 is a section view showing a traditional mud cap drilling system. After a borehole is drilled, a casing 30 is disposed therein and cemented in the wellbore 15. A drill string 35 is disposed in the wellbore 15 creating an annulus 10 between the casing 30 and the drill string 35. The drill operator loads the annulus 10 by pumping a predetermined amount of heavy density fluid in an inlet port 60. This fluid is designed to minimize gas migration up the annulus 10. After the fluid reaches the predetermined hydrostatic pressure, the drill operator shuts in an inlet port 60.
As illustrated on FIG. 1, the system includes a rotating control head (RCH) 50 at the surface of the wellhead 15. The RCH 50 includes a seal that rotates with the drill string 35. The heavy density fluid applies an upward pressure on the downward facing RCH 50, thereby sealing off the outer diameter of the drill string 35. The purpose of the RCH 50 is to form a barrier between the heavy density fluid mudcap and the rig floor. At this point, the shut in surface pressure on the annulus plus the hydrostatic pressure resulting from the heavy density fluid equals the formation pressure. This annular column of heavy density fluid is held in place by a pressure barrier 45 created between hydrostatic fluid column pressure and the downhole pressure. To offset any annular loses of fluid into to the formations 25, it may be necessary to add fluid to the mudcap in the same sequence as it was initially introduced. Additionally, the system also includes a blow out preventor 55 (BOP) disposed at the surface of the well for use in an emergency. Thereafter the mudcap is established, the drilling operation may continue pumping clean fluid that is compatible with the formation fluids down a drill string 30 exiting out nozzles in a drill bit 40. A permeable formation fracture 25 receives the drilling fluid as it pumped down the drill string 30. A term used in the oil and gas industry called xe2x80x9cbullheadingxe2x80x9d results due to the formation of the barrier 45 at the bottom of the annular column 10 between the heavy density fluid and hydrocarbon formation pressure. The barrier 45 prevents drilling fluid returning to the surface, thereby urging the fluid into the formations 25. Although this process requires specialized well control and well circulation equipment during the mudcap drilling operation, there is no need for extensive fluid separation system since the formation fluids are kept downhole.
There are several problems that exist with the traditional mudcap drilling system. For example, as with other forms of well control the surface rotating control head (RCH) is the only barrier between the high-pressure return fluid and personnel on the rig floor. The operators are often concerned about safety on high-pressure wells since there is no early warning system in place. In another example, the RCH stripper rubbers wear out rapidly due to the high differential pressure. These stripper rubbers need to be changed periodically on the job to ensure proper functioning of the RCH. This is a costly operation in terms of rig time and cost of the rubber elements. In a further example, this drilling method can only operate if a permeable fracture or formation exists because all the drilling fluids are not returned to the surface but are being pumped into a permeable fracture. This drilling fluid loss is also a costly investment. In yet a further example, reservoir damage can occur due to the lack of control of a true underbalanced state between the fluid column pressure and the formation pressure, thereby reducing the productivity of the well. In the final example, the well does not produce hydrocarbons while tripping the drill string in a traditional mudcap drilling operation.
In view of the deficiencies of the traditional mudcap drilling system and other well control methods, a need exists to ensure the safety of the rig operators by providing an early warning system to tell the operators that a potential catastrophic problem exists. There is a further need to extend the life of the RCH due to the high cost of non-productive rig time as a result of replacing the rubber part. There is yet a further need to save operational costs and prevent formation damage by allowing the drilling fluid to return to the surface of the wellhead while maintaining the benefits of a traditional mudcap system. There is yet even a further need for a mudcap assembly, which allows the well to produce hydrocarbons while tripping the drill string.
The present invention provides a method and an apparatus for a dynamic mudcap drilling and well control assembly. In one embodiment, the apparatus comprises of a tubular body disposable in a well casing forming an outer annulus there between and an inner annulus formable between the body and a drill string disposed therein. The apparatus further includes a sealing member to seal the inner annulus at a location above a lower end of the tubular body and a pressure control member disposable in the inner annulus at a location above the lower end of the tubular body.
In another embodiment, the assembly uses two rotating control heads, one at the top of the wellhead assembly in a conventional manner and a specially designed downhole unit. Thus, creating dual barriers preventing any potential leak of produced gases or liquid hydrocarbon on to the rig floor, thereby ensuring the safety of the rig operators. Furthermore, the assembly provides an early warning method for detecting catastrophic failure in any of the two rotating control heads. Additionally, the assembly provides a practical method for reducing wear on the RCH stripper rubbers by ensuring the pressure differential across both the surface and downhole RCH is small, thereby extending the life of the RCH and reducing the non-productive time of the rig due to periodic replacement of the rubber part in the RCH. Further, the assembly provides for a way of circulating the return flow to the top of the wellbore thereby reducing cost of drilling by utilizing the return drilling fluid. Further yet, the assembly provides a practical method for containing and controlling wellhead pressure of return fluids by use of a high-density fluid column. Additionally, the assembly using a WEATHERFORD(copyright) deployment valve allows the well to continue to produce hydrocarbons without any drill string in the well bore. Finally, the assembly provides a method for allowing the well to produce hydrocarbons while tripping the drill string.